190 F W. Clarke — Tschermak's Theory of the 



Art. XXIII. — Tschermatts Theory of the Chlorite Group and 

 its Alternative ; by F. W. Clarke. 



In the micas, vermiculites, the clintonite group, and the 

 chlorites, we meet a large number of minerals having similar 

 properties. All are probably monoclinic, all are commonly 

 foliated in structure, and within certain limits they shade into 

 each other in composition. The resemblances between them 

 are very close externally ; and it is therefore a fair presump- 

 tion that they are also alike in constitution. A satisfactory 

 explanation of their chemical structure ought to emphasize 

 their obvious relationships, and be sufficiently general to in- 

 clude all the individual species in a single definite theory. 

 The problem is easily stated, but its solution can be effected 

 only by slow degrees. Like other similar problems, it must be 

 solved by, a series of approximations ; and every step towards 

 order is a gain, even though it be not absolutely final. 



Among the men who have advanced our knowledge of these 

 minerals, no one is entitled to higher credit than Professor 

 Tschermak. His successive papers upon the mica, clintonite, 

 and chlorite groups have all been long strides forward ; and 

 whether his interpretations are ultimately confirmed or aban- 

 doned, they have none the less been of real service in the 

 development of knowledge. 



In his latest paper,* Professor Tschermak seeks to explain 

 the chemical structure of the chlorites, and masses the availa- 

 ble evidence most admirably. This group of minerals he 

 divides into two sub-groups, called orthochlorites and lepto- 

 chlorites respectively, and for each division a special explana T 

 tion is offered. The orthochlorites, which include pennine, 

 clinochlore, leuchtenbergite, prochlorite and corundophilite, 

 are simply represented as mixtures of two end products, ser- 

 pentine and amesite ; and so far as the mere ultimate composi- 

 tion of the minerals is concerned, the interpretation is quite 

 satisfactory. On theoretical grounds it leaves something to be 

 desired, as will appear later. The leptochlorites, however, or 

 chlorites outside of the main series, are less easily handled ; 

 and for these, in addition to the serpentine and amesite mole- 

 cules, four other fundamental compounds are assumed ; namely, 

 two derivatives of amesite, strigovite and chloritoid. Six types 

 of molecule are in all adopted ; which, by mixtures in various 

 proportions account for all the species. Written semi-empiri- 

 cally, these molecules are as follows : 



* Die Chloritgruppe, II, Theil, Sitzungsb. Akad. d. Wiss, in Wien, 19 Feb., 1891. 



